In recent years, phononics, the discipline of science and technology
in processing information by phonons and controlling heat flow,
becomes more and more exciting [1]}, [2]}.
Various functional thermal devices such as thermal diode [3]},
thermal transistor [4]},
thermal logic gates [5]} and thermal memory [6]},
etc., have been proposed to manipulate and control phonons, the
carrier of heat energy and information. And very recently, similar to
the Hall effect of electrons, Strohm et al. observed the phonon
Hall effect (PHE) – the appearance of a temperature
difference in the direction perpendicular to both the applied
magnetic field and the heat current flowing through an ionic
paramagnetic dielectric sample [7]}, which was confirmed
later in Ref. [8]}. Such observation of the PHE is really
surprising because phonons as charge-free quasiparticles, different from
electrons, cannot directly couple to the magnetic field
through the Lorentz force. Since then,
several theoretical explanations have been proposed
[9]}, [10]}, [11]}, [12]} to understand this novel phenomenon.
From the work of the PHE in four-terminal nano-junctions and
the phonon Hall conductivity in the two-dimensional
periodic crystal lattice, we know that the PHE can exist even in
the ballistic system.
